re-add scalameta to 2.13 community build

[SethTisue]

@xeno-by you can see the failure details at https://scala-ci.typesafe.com/job/scala-2.13.x-integrate-community-build/318/consoleFull — it's because scala-parallel-collections is a module now. see https://github.com/scala/scala-parallel-collections#usage (which I just wrote today) for instructions on how to deal with it in your build and code

this isn't necessarily urgent, since (even in the 2.12 build) scalameta's only dependent project is scalafix. kinda up to you whether you want early notice of when Scala 2.13 changes break scalameta, or if you'd rather deal with it later all at once

[SethTisue]

closing because tons of projects have been removed from the 2.13 build, we don't need individual tickets